Electrical-heater resistance element



se fs Patented July 1, 1 924.

CROSS REFERENCE um'reo sures PATENT OFFICE.

30m! WOOLBIDGI REYNOLDS, OI DABLINGEUBST, m3 mm, NEW SOUTH WALES, AUSTRALIA.

ELECTRICAL-HEATER BEIB'IANCI M.

Io Drawing.

' 10 the following is a s cification.

According to a own process electrical heater resistance elements are constituted of a film of a paint like substance which is applied to a support consisting of a heat toler- 15 ant electric msulating material, by painting it on said material and allowing it to dry upon it in situ.

' powdered This paint is composed of chemically precipitated 'licates portion of sodium silica/afiel 9 0 tassium silicate. use quite sat1s actory service is obtained and the filnis are notably permanent in their character, showing no tendencv to disintegrate or to shell or fret away from the support. They maintain; substantially uniform resistance characteristics while in continuous or intermittent use, but they are faulty in one minor respect- When the elements are placed in store or stock for a prolonged period the electrical resistance of the paint film in- A creases, thereby affecting the heating effect of the element. This hlgh resistance gradually diminishes while the element is in use until it reaches the normal int, and thereafter it remains substanti y constant, and the heating value likewise remains substant1al ly constant so long as the element is mamtained in continuous or intermittent It' has, therefore, been found desirable to devise a means wherebghtghis augrnentation in resistance of the during idle periods may be prevented, and the present invention consists in means for curing the fault referred to so as to invest the painted film with approximately constant electncal resistance value, and co uently ensure that the heating value of t e element will remain substantially uniform whether the unit be maintained in use or stored away.

The tendency of the painted film to augment its resistance when out of use can be Application Med January 11, ma Serial n. mas-IQ 7 I dium silicate or potassium silicate m it to a low point, but even then the faul condition will not be sufiiciently remov to satisfy commercial requirements, and there is a mt below which it is not advisable to reuce the proportion of sodium silicate or potassium silicate without running the risk of enda rin the mechanical stability of the film, m w ch case it would no longer remain permanently attached to its support and resist scaling and fretting satisfactorily. It is highly desirable to maintain the pro rtionof sodium silicate or potassium s' 'cate in the composition at about normal value toensure quite satisfactory mechanical permanency of the film.

It has been ascertained experimentally 70 that if films prepared and ap lied in the known manner, are treatedwit dilute sulhuric acid or dilute h drochloric' acid after Elie d g process has "jfco'm'pleted, an are an uentl washed with warm water, II the objectlonab e behaviour of the film in res t of its electrical resistance is prevent- For this .purpose the films are washed with the acid solution and after such treatment are thoroughly washed m n'mmng water. The films are thus set, and retain permanently the initial electrical resistance values which they display when first dried out. I

When the treatment of thefilmwith dilute acids is practised it is found that in the making up of the film composition a higher proportion of added silicate of soda or silicate of potash. may be used than is otherwise practicable, with the result that greater terlizgity and longer life of the film 1s as- In the manufacture of film composition according to this invention, an a ueous so- Eli Tt'ifile lution of ul hate, chloride oro or m salt of aluminium is added to an aqueous sou ion 0 commercial sodi silicate. A white gelatinous preci itate is t us produced. The addition 0 the aluminium salt is continued only so lon as recipitation continues to take place; t erea r the precigiiate is permitted to settle for ten to 6 n minutes, more or less. The bulk of the supernatant liquor is then removed by siphomng or otherwise, and very fine 119m reduced by reducing the proportion of soggphite and a minor proportion of tmmmt silicate are added and thoroughly incorpora e with the precipitate y mixing whilst wet. v

The proportions of the substances used are I in the following ratio or thereabouts:

1 re su stance thus constituted is apphed to a heat tolerantsupport suchas mica or a fire-enamelled metal sheet or plate, care being taken to ensure that the electrical resistance of the enamel is sufliciently high to avoid wasteful leakage. The paint is applied to the support by means of a soft brush to form a skin or film on it, and contacts offering a lar surface are a plied to the terminals of t e painted-on After the film is thoroughly dry it is submitted to the 06858 in which this invention consists, t is to say it is freely washed in a dilute (i. e. 10% more or lea) aqueous solution of sulphuric acid or hydrochloric acid once or oftener, and immediately after the acid washing it is rinsed thoroughly several times in runnin water. The film is then again allowed to when dry is found to have constant electrical resistance, so that even after prolonged cessation of use its utility is not diminished, and a period of delay in use is not, as heretofore, encountered in waiting for the film to develo its normal resistance.

' What claim as my invention and desire to secure by Letters atent is 2- p The herein described improvement in the manufacture of electrical resistance films composed of preci itated siliceous material, graphite, and an a aline silicate, which consists in washing said films after they are set on their supports with a dilute solution of sulphuric acid or hydrochloric acid and subsetillilently washing them with water.

ry, and

testimony whereof I afiix my signature.

sown woommcs nsmows. 

